The present invention relates to crystal-stabilized integrated circuit oscillators.
Crystal-controlled oscillators use the high Q of an electromechanical resonator (a quartz crystal) to stabilize an integrated oscillating circuit at a desired frequency. Such circuits can achieve a frequency stability in the parts-per-million range, and there is no other practical way to achieve such a constant frequency reference in an integrated circuit. Crystal-controlled oscillators are therefore extremely important, and likely to remain so.
Crystal-controlled oscillators pose some difficulties in design, and one of these is start-up. The impedance of the crystal is typically much higher at zero current condition (which are necessarily present at start-up), so in come implementations the initial loop gain is not enough to start the oscillator. A variety of startup circuits have therefore been proposed; see e.g. B. Parzen, DESIGN OF CRYSTAL AND OTHER HARMONIC OSCILLATORS (1983), at page 415: Unkrich et al., "Conditions for Start-Up in Crystal Oscillators," 17 IEEE J. Solid-state Circuits 87 (1982).
Other difficulties are present in the specific context of low-power CMOS oscillator implementations. Many portable applications are designed for low operating voltage and low power consumption, but also require the frequency stability of crystal oscillator. To reduce power consumption, such low-power CMOS oscillator circuits are typically operated in the weak inversion regime (where gate voltages are only slightly greater than the threshold voltage). However, in the weak inversion regime the gain tends to be lower, and thus start-up is a particularly critical problem. See e.g. U.S. Pat. No. 5,546,055, which is hereby incorporated by reference.
Many circuits have been proposed to permit the loop gain to be increased at startup. The input to make this change is conventionally supplied by some external circuit. A prior art example of this is shown in FIG. 4 (taken from U.S. Pat. No. 4,896,122), where the "programmable activity detector" 402 is digital circuit. Until this digital circuit detects the presence of logic transitions on the output FO of final amplifier 322, it generates a logic signal BWC. When this signal is active (and external control signal OSON is active), auxiliary gain stage 218 is turned on, and operates in parallel with the primary gain stage 202. However, the use of a digital circuit to make this determination provides only imperfect matching to the variations which may affect the characteristics of the analog oscillator.
Innovation Structures and Methods
The present application discloses a crystal oscillator which uses a filtered analog coupling to automatically disable the bias current to an auxiliary gain stage after startup. Preferably this analog coupling includes some positive feedback, to ensure that the switchover is completed once it starts. Thus the device sizes and biases of the primary gain stage can be selected for very low-power operation, while assuring that the oscillator will always start-up whenever power is valid.
Advantages of the disclosed methods and structures include:
reliable oscillator start-up: PA1 robust design which is tolerant of minor variations: and PA1 minimum-power operation.